Sun May 04, 2014 9:15 pm
So I have decided to get Betty Lou's engine squared away cause she was smoking bad. I am at the point of taking measurements now. All pistons are STD. The service manual calls for the piston diameter to be 2.6230 - 2.6234. I measure my pistons and they are 2.655. Standard Bore is 2.625 to 2.627. My bores are measuring out at 2.665 to 2.668. Heres is the question. How is this possible? I am measuring with a dial caliper and bore gauges.
EDIT. The PO had been through the engine recently, because things are way too clean. I am assuming they re-ringed without honing/boring. The rings look new and there wasnt even a ridge at the top of the cylinders
Sun May 04, 2014 9:45 pm
Block has been bored out .030"
Sun May 04, 2014 9:47 pm
.030 over size
Sun May 04, 2014 9:49 pm
Eugene wrote:Block has been bored out .030"
1. Why would someone go through the work of boring an engine and then put standard pistons in.
2. Why are the pistons (STD) .030 larger than what the book specs as STD?
Sun May 04, 2014 10:09 pm
do the pistons have any stamps on top, such as A, B, C, or D? are any numbers stamped on them? also, those measurements should be made with a micrometer. Dial calipers are handy to have, but not normally all that accurate.
Sun May 04, 2014 10:24 pm
John *.?-!.* cub owner wrote:do the pistons have any stamps on top, such as A, B, C, or D? are any numbers stamped on them? also, those measurements should be made with a micrometer. Dial calipers are handy to have, but not normally all that accurate.
I know there are some numbers ( other than STD) stamped on top of the pistons, but I will have to look at them again. The block was marked " A B B A " and the #2 and 3 cylinders have had their "b" crossed out and an "a" stamped in its place. I agree that calipers are not as accurate as a Micrometer, but .030 is a HUGE difference in the world of precise measurement. I wouldnt think they would be off that much.
EDIT- The pistons have STD A, and then to one side it says AP 137
Sun May 04, 2014 10:41 pm
The letters stamped in the block are factory bore measurements and have nothing to do with current bore measurements.
You pistons are .030 over standard. Do the math. Subtract standard piston size from current piston size. Do the same for current bore size and standard bore size. You will find a small bit of difference in piston size since they are actually a tad bit smaller than the bores.
Edit: Just reading the numbers provided in the initial post.
Mon May 05, 2014 9:40 am
I will have a piston measured with a micrometer this afternoon to see if my dial caliper is the culprit like John mentioned. Hopefully it is off by exactly .030 as the measurements suggest.If my caliper is correct, then I will be on the hunt to figure out why a piston marked STD is actually .030 oversize.
Mon May 05, 2014 8:08 pm
Wonder if standard pistons from some other engine might be .030 oversize for a cub. I think some Continental engines may have been close in size. Might want to measure the size of the wrist pin also.
Mon May 05, 2014 8:18 pm
Another possibility is that someone put 0.030" oversize rings on standard pistons. Would run a little loose and risk scuffing, but at cub speeds, might run a long time with minimal issues.
Mon May 05, 2014 8:21 pm
John *.?-!.* cub owner wrote:Wonder if standard pistons from some other engine.
My thought also.
You might want to find out the source of the pistons - - - or when you are ready to rering them, measure the lands and grooves. Making sure the new rings fit the pistons.
Tue May 06, 2014 2:35 pm
I measured them with three different calipers. I could not get my hands on a micrometer. All three measured the same. I will attach pics if I can figure out how on this fartsmone...
Tue May 06, 2014 4:44 pm
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Tue May 06, 2014 8:21 pm
M & W are common after market pistons. Don't know where IH purchased or if they manufactured their own pistons.
If you are purchasing new rings, you need to remove the expander in the oil control ring groove.
Tue May 06, 2014 9:33 pm
You have just answered the questions, those are aftermarket pistons, and I suspect if you go to an M&W site you will find AP137 to be the part number for a .030 oversize.
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